VTOL aerial vehicles generally fly thanks to a propulsion system (e.g. one or more propellers) that generates an upward force (lift) to counter gravity. Such vehicles are capable of slow flight (hovering flight), vertical take-off or vertical landing, and have generally a control system to control their orientation or direction in order to stay in a stable orientation or to move sideways. When the aerial vehicle is not in a stable orientation, e.g. its propulsion system creates a force that is not pointing mostly upwards, the aerial vehicle can quickly lose lift, or gain speed towards the direction in which the propulsion system creates a force.
VTOL aerial vehicles that stay aloft using a propulsion system and a control system exist in several configurations known in the prior art. Examples of such VTOL aerial vehicles comprising a propulsion system and a control system are illustrated in FIG. 1(a) through FIG. 1(d) for reference (many more types exist but are not illustrated in the Figs.). The propulsion system usually comprises one or more propellers 101, and provides the lift force that keeps the aerial vehicle in the air. The control system can take various forms and usually comprises some control electronics 102 and possibly additional actuators not used to generate lift, but rather to generate forces or torques around the roll 110, pitch 111 or yaw 112 axis, in order to control the roll 113, pitch 114 and the yaw 115 motions, or in other words the aerial vehicle's orientation and thus the direction of the lift force. Such aerial vehicles stay in the air by remaining in an orientation where the propulsion system generates a force that is mostly upwards (usually by having the propellers rotate approximately in the horizontal plane), and they move sideways by slightly tilting.
FIG. 1(a) illustrates a multi-rotor system, and specifically a quadrotor is illustrated. Such an aerial vehicle uses a propulsion system comprising several horizontal propellers 101 generating lift. The control system determines the speed of each individual propeller to stabilize the aerial vehicle in a stable orientation or to tilt it so that it moves sideways. The differential actuation of opposite propellers generates torques around the pitch and roll axes. The differential actuation of propellers turning in opposite directions generates a torque around the yaw axis.
FIG. 1(b) illustrates a typical helicopter propulsion system. This aerial vehicle uses one main horizontal propeller 101 as a propulsion system to generate lift. The control system controls the pitch and roll motions of the aerial vehicle thanks to a swash-plate 104 that actuates the pitch of the main propeller's blades and stabilizes the pitch and roll of the aerial vehicle, or tilts it to move sideways. A vertical tail propeller 103 is used to control the yaw angle.
FIG. 1(c) illustrates a co-axial design with fly-bar. Such an aerial vehicle uses two horizontal propellers 101 rotating in opposite directions as a propulsion system. The control system comprises a fly-bar 105 that keeps the pitch and roll angles stable. The fly-bar is a rigid rod with a relatively high moment of inertia that rotates together with the top propeller and remains horizontal thanks to inertia. It is mechanically linked to the pitch of the upper propeller's blades so that when the aerial vehicle's orientation is disturbed, the propeller creates a torque that brings back the aerial vehicle in a stable orientation. The differential actuation of the two propellers allows control of the yaw angle. The lower propeller can be equipped with a swash-plate in order to control the pitch and roll motions, and thus move the aerial vehicle sideways.
FIG. 1(d) illustrates a co-axial design with control surfaces. This aerial vehicle uses two horizontal propellers 101 rotating in opposite directions as a propulsion system. The control system uses one pair of control surfaces 108 to control the pitch motion and another pair of control surfaces 106 to control the roll motion. The control surfaces are actuated by two actuators 107, and generate forces by deflecting the airflow generated by the propulsion system. The differential actuation of the two propellers allows control of the yaw angle.
Aerial vehicles designed to fly close to obstacles, such as disclosed in EP2517767A2 “Self-righting frame and aeronautical vehicle” to J. Dees and G. Yan, are often equipped with protective structures typically surrounding the propulsion system and control system. These protective structures prevent external objects from damaging sensitive parts such as rotating propellers or control surfaces, or absorb collision energy when the aerial vehicle collides into obstacles or falls to the ground. They are generally built so that openings allow the airflow to go through the structure without affecting too much the lift force generated by the propulsion system. The shape of the protective structures can be designed so that the aerial vehicle will upright passively to a vertical take-off orientation when it lies on flat ground.
A few existing VTOL aerial vehicles use protective structures with moving parts in order to improve the interaction with the environment. As described in M. Itasse, J.-M. Moschetta, Y. Ameho, and R. Can, “Equilibrium Transition Study for a Hybrid MAV,” International Journal of Micro Air Vehicles, vol. 3, no. 4, pp. 229-246, December 2011, a dual-motor VTOL is equipped with two passively rotating wheels that both protect the rotors from contact, and can be used to roll on the ground or even along the wall when in flight. However, the wheels can only protect the inner frame of the aerial vehicle from touching flat obstacles, and the aerial vehicle can only roll toward a single direction.
In A. Kalantari, and M. Spenko, “Design and Experimental Validation of HyTAQ, a HybridTerrestrial and Aerial Quadrotor,” IEEE International Conference on Robotics and Automation, 2013, a protective cage is described that can passively rotate around one axis and offers better protection, which allows the VTOL aerial vehicle to roll on uneven ground towards a single direction only. While these aerial vehicles demonstrate passively rotating protective structures for rolling on obstacles, their rolling direction is constrained to a single direction because of the single axis of rotation. Also, among other things, these mechanisms do not address the problem of reducing disturbances occurring from in-flight collisions with obstacles. An aerial vehicle described in US2010/0224723A1, “Aerial Vehicle” to J. Apkarian, features a protective structure that can rotate around two different axes. However, the rotation axes are fully actuated and controlled at all time by motors. Among other things, the actively controlled rotation of the structure only allows the stabilization of the aerial vehicle during flight by changing the position of the center of mass of the protective structure. This design does not allow for disturbance reduction when colliding in flight with obstacles, or for rolling on obstacles (e.g. in contact with obstacles during flight).
U.S. Pat. No. 6,976,899 to Tamanas describes an “all terrain vehicle” comprised of three connected rings, a cradle attached to the innermost ring and configured such that the vehicle rolls upon the ground while the cradle remains upright. Among other things, while the all terrain vehicle is configured to travel over various ground surfaces, the all terrain vehicle is not configured and has no application as a vertical take-off and landing aerial vehicle.
When an aerial vehicle enters into contact with an obstacle, relatively large external torques and forces can disturb the orientation of the aerial vehicle. While an onboard control system (mechanical and/or software) might counter some amount of disturbances and bring back the aerial vehicle in a stable orientation for flying, such control systems are often unable to correct quickly the large disturbances occurring after a contact with external objects. Such contacts can thus provoke large perturbations of the aerial vehicle's orientation or trajectory, or even lead to a crash to the ground. Most aerial vehicles are thus always kept away from obstacles, to prevent any contact with obstacles. Additionally, most aerial vehicles can only take-off from one resting orientation, in which the propulsion system can create an upwards force, which limits their ability to take-off from uneven ground, or after landing in other orientations.